In a prior DS-SS communication system, a code division multiple access (CDMA) system has been used, in which each station is assigned a distinct pseudo noise code (PN code), and each station is recognized based upon the specific PN code, and the desired signal is derived based upon the fact that the correlation between the codes of PN code is small.
FIG. 8 shows a block diagram of a prior CDMA system, in which k number of stations may communicate at the same time. The numerals 100-1 through 100-k are an information signal in digital form of each station, the numerals 101-1 through 101-k are a multiplier for multiplication of an information signal 100-1 through 100-k, and a pseudo noise code (PN code) 102-1 through 102-k, the numerals 102-1 through 102-k are a pseudo noise code for spread spectrum of an information signal 100-1 through 100-k, the numerals 103-1 through 103-k are a modulator for modulating a base band signal obtained from the multipliers 101-1 through 101-k with the carrier frequencies obtained by the local oscillators 104-1 through 104-k, the numerals 105-1 through 105-k are a band pass filter for deriving spectrum component necessary for transmission from the modulated signal.
The spread spectrum communication system may be used both in a wired communication system, and a radio communication system. The SS (Spread Spectum) signals of each stations are multiplexed in transmission medium, and the multiplex process is equivalently expressed by an adder 106 in FIG. 8.
The numeral 107 is a band pass filter for deriving a frequency component necessary for demodulation from a receive signal, the numeral 112 is an AGC (automatic gain control) for providing constant level of an output of the filter 107, the numeral 108 is a frequency converter for converting a received high frequency to an intermediate frequency, the numeral 109 is a local oscillator for the frequency conversion, the numeral 110 is a demodulator for detecting an information transmitted by each transmit station in a frequency restricted receive signal, and the numeral 111 is a received information signal obtained by the demodulator 110.
In the prior art of FIG. 8, the information signal of each transmit station is directly spread spectrum modulated by using pseudo noise codes PNl through PNk which have small correlation with one another. The local frequency of each local oscillators 104-1 through 104-k is the same as f1The modulated signals by the modulators 103-1 through 103-k are frequency restricted by the band pass filters 105-1 through 105-k, and are transmitted towards a receive side.
A receive station receives the multiplexed signals 1 through k with common carrier frequency f1 as shown in FIG. 9. A demodulator obtains only a desired signal through correlation detection which uses the same pseudo noise code as that which is used in a transmit station for spreading an information signal.
In a DS-SS communication system, a maximum length sequence (M sequence) is generally used as a pseudo noise code for spectrum spread direct modulation. However, the number of M sequence is 18 for a pseudo code with code length 127, 16 for the code length 255, or 48 for the code length 511. Therefore, in a CDMA communication system which discriminates a transmit station by a pseudo noise code must use an M sequence with long code length so that many stations may transmit simultaneously. However, the use of a pseudo noise code with long code length for many simultaneous transmit stations complicates the generation of the code, the structure of a correlator, and the process for demodulation.
When more stations transmit at the same time, the transmit frequency must be changed as shown in FIG. 10, since the same pseudo noise code must not be used with the same carrier frequency. FIG. 10 shows the example that the first group of the stations 1 through k uses the carrier frequency f1, and the second group of the stations 1 through k uses the carrier frequency f2. When m times of the stations are desired to transmit simultaneously, in other words, each pseudo noise code is used m times repetitively, the frequency band must also be m times, therefore, if the frequency band to be used is restricted, the number of stations for simultaneous transmission is then restricted.
One modification of a prior art is to assign a pseudo noise code to a transmit station upon request of the transmit station, instead of assigning a fixed pseudo noise code to each transmit station. However, this system is complicated in control and an apparatus.
One prior solution for solving the above problems is CFO-SSMA system (Carrier Frequency Offset-Spread Spectrum Multiple Access Method) which is described in JP patent laid open publication 268189/1993, and U.S. Pat. No. 5,319,672. In that system, a plurality of transmit stations use a common pseudo noise code for spread spectrum modulation for independent transmit signals, and the center frequencies of each transmit signals are offset as shown in FIG. 11, so that each frequency band of each transmit signal overlaps with one another. When each transmit station uses a carrier frequency which is offset from one another by the frequency equal to integer multiple of signal transmission rate, then, a receive side can derive a desired signal by using a matched filter in association with transmitted SS signal (PN code), and may demodulate the signal with no interference by other signals.
The CFO-SSMA system which uses a carrier frequency offset from one another by an integer multiple of signal transmission rate has the advantage that each transmit station may transmit a signal with no interference with another station even when the same pseudo noise code is used in a spread spectrum communication system. Therefore, even when a number of pseudo noise codes is restricted, each pseudo noise code is re-used by shifting a center frequency of each transmit station by an integer multiple of a transmission signal rate, and thus, a number of stations which communicate simultaneously is considerably increased in a restricted frequency band. Further, as each transmit station uses the same pseudo noise code, an apparatus may be simple and small.
However, the CFO-SSMA system has the disadvantage that the communication with no interference is possible only when the timing of the spread spectrum modulated signals transmitted simultaneously coincides with one another in a receive side. In other words, when a spectrum spread modulated signal is asynchronous in a receive side, a signal from a transmit station suffers from interference with another signal, and the signal quality is decreased.
Further, when an AGC (automatic gain control) is used for providing constant receive level, the receive level of each signal depends upon a number of signals, since an AGC controls a receive level constant irrespective of a number of multiplexed signals. Therefore, when a receive station regenerates a clock timing through the comparison of a peak value of a correlation of a desired signal with a predetermined threshold, a receive level of a desired signal when many signals are multiplexed would be decreased because of the operation of an AGC, and the peak value of the correlation would be lower than the threshold. In that case, the signal reproduction would be impossible. In order to solve the above problem, the threshold level must be changed with a number of multiplexed signals, however, the implementation of that would complicate an apparatus, and therefore, it is not practical solution.
As described above, a multiple access spread spectrum communication system using a common pseudo noise code in all the transmit stations has the disadvantages that each spread spectrum modulation signal would interfere with each other in a receive side unless a timing of all the signals coincides with each other in a receive side, and causes the deterioration of signal quality of a transmission circuit, although it has the advantage that the restricted frequency band is used with high efficiency, and a simple communication apparatus may be used.
A synchronization among stations would solve the above problem through the communication between a plurality of transmit stations and a plurality of receive stations, however, the protocol for high precision synchronization would be complicated, and an apparatus for that would be complicated.